Low rev hesitation?

MattW · Mar 19, 2011

After about three months unused over winter, I've just reassembled my '94 1100 after having had the throttle bodies re-bushed. I fitted a new throttle cable at the same time and balanced everything up.

I noticed when balancing at tickover that there was a slight unevenness - a sort of hesitation every second or so giving a lumpy idle. It ran smooth at 3000 rpm for the high rev balance.

Now having ridden it, I notice the same thing on the road. At low revs it doesn't run smoothly - it's not desperate but it hesitates every so often as if there's a momentary interruption in the power. It doesn't feel like surging (it's never suffered with this) - definitely feels like a split second hesitation. At higher revs, it's fine and pulls as well as ever (the difference in running with the refurbished, freshly balanced TB's is great - turbine smooth at cruising revs).

I fitted a new hall sensor plate last year and its valve clearances are spot on.

Anyone got any ideas as to what might be causing this?

MattW · Mar 19, 2011

Well, I guess it'll be ignition related so I suppose I'll start with plugs and work back. Just looked at the price of HT leads :eek

MattW · Mar 19, 2011

Anyone any idea what's more likely to be at fault? HT leads, coil?...
Speculative swapping could soon get expensive.

jmh · Mar 19, 2011

Hi, mine has been doing similar after a winter lay up, been striped down for crank seal etc. just seemed to miss now an then at idle, tried some injector cleaner, seems to have done the trick for mine! If you think it's HT leads you could try running it in the dark an look for sparks! I've had that on other bikes. Goog luck.

MattW · Mar 20, 2011

jmh said:
Hi, mine has been doing similar after a winter lay up, been striped down for crank seal etc. just seemed to miss now an then at idle, tried some injector cleaner, seems to have done the trick for mine! If you think it's HT leads you could try running it in the dark an look for sparks! I've had that on other bikes. Goog luck.

Ta - no harm in trying injector cleaner I guess :thumb2

MattW · Mar 20, 2011

Well, looks like I'm on my own with this

I might try and come up with a way of seeing the ignition pules at the HT leads with my oscilloscope (I'll have to make some sort of inductive coupler). Although even if it works, whether I'll be able to differentiate a weak pulse from a normal one is anyone's guess.

uncle dick · Mar 20, 2011

Have you tried adjusting the co pot to see if it helps? On my '96 I set it to factory settings with a good co meter and, to be honest, it was crap. I richened it a turn at a time and am now at 3 turns ccw from the 'correct' setting. I also found that a change from original - BCP7ET - plugs to Iridium replacements helped a lot at very low rpm.

Also, make sure that your idle bypass screws are roughly equal. If you've had the bushes done, the initial butterfly settings may now be out. If the LH butterfly is set and the throttle pot voltage ok, set the RH bypass screw the same as the LH one and adjust the RH butterfly to balance up. Then set the cable as usual.

I know people say 'never touch the butterfly screws' like it's some kind of black magic but if you don't do anything it'll stay the same.

I'd be surprised if it's a HT fault.

HTH
Dick

MattW · Mar 21, 2011

uncle dick said:
Have you tried adjusting the co pot to see if it helps? On my '96 I set it to factory settings with a good co meter and, to be honest, it was crap. I richened it a turn at a time and am now at 3 turns ccw from the 'correct' setting. I also found that a change from original - BCP7ET - plugs to Iridium replacements helped a lot at very low rpm.

Also, make sure that your idle bypass screws are roughly equal. If you've had the bushes done, the initial butterfly settings may now be out. If the LH butterfly is set and the throttle pot voltage ok, set the RH bypass screw the same as the LH one and adjust the RH butterfly to balance up. Then set the cable as usual.

I know people say 'never touch the butterfly screws' like it's some kind of black magic but if you don't do anything it'll stay the same.

I'd be surprised if it's a HT fault.

HTH
Dick

Thanks for the advice Dick - very helpful.
I've not tried adjusting the CO pot - not having a CO meter means I've always left it alone. I've a suspicion it's running a bit rich at tickover (sooty tailpipe) but overall the fuel consumption is good (50+ mpg) and it runs really well so I've not adjusted anything.

The idle air screws are about 1/2 turn different - it's been like this from when I got the bike three years ago and the rebushing hasn't appreciably changed the settings. I must admit, I've listened to the advice regarding 'don't touch the throttle stops' and I've left them alone. I guess this is the reason that it's never had a totally smooth idle (even freshly balanced), however the little hesitations I'm currently experiencing are new. The throttle stop screws still have their little paint blob seals on them so I'm pretty sure they've never been messed with.

One thing I've not thought of checking until just now is the TPS - it seemed fine when I reassembled the TB's and set the voltage but I'll put a meter across it with the bike running and make sure it's not throwing any spurious readings at tickover.

I've actually just bought some iridium plugs - I'll fit them tonight and run some injector cleaner through it and see what happens.

thanks again for the advice

Matt

Garry H · Mar 21, 2011

I have a simular problem with my 1100 too :blast

It's been like it for the last 2 years...I have tried to sort it out with varying degrees of success
But have never got the running spot on...
Over that time i have replaced the leads, TPS, and ignition coil
I have also done the valves (more than once) stripped and cleaned the throttle bodys adjusted the CO pot (with a meter) removed and inspected the hall sensor checked the other sensors..
I have also spoken to the local Bm dealer who says it's a common problem and advised me to remove the cat code plug...That helps but kills the fuel economy..
Trouble is it spoils the bike so i am now in the process of ditching the fuel injection and fitting carbs...
If i can get it to run as sweet as my airheads i will be happy
Good luck with yours :comfort

MattW · Mar 21, 2011

Garry H said:
I have a simular problem with my 1100 too
It's been like it for the last 2 years...I have tried to sort it out with varying degrees of success
But have never got the running spot on...
Over that time i have replaced the leads, TPS, and ignition coil
I have also done the valves (more than once) stripped and cleaned the throttle bodys adjusted the CO pot (with a meter) removed and inspected the hall sensor checked the other sensors..
I have also spoken to the local Bm dealer who says it's a common problem and advised me to remove the cat code plug...That helps but kills the fuel economy..
Trouble is it spoils the bike so i am now in the process of ditching the fuel injection and fitting carbs...
If i can get it to run as sweet as my airheads i will be happy
Good luck with yours

Thanks Garry

Trullion · Mar 21, 2011

My 850GS used to "miss" every now and again when idling and also used to run rich. I cured it by careful setting of the pot and the tps. Both the tickover and tps should be adjusted so that there is a smooth transition from tickover to main throttle. IMHO this is more important than a specific voltage for the tps setting, as long as it's in the working range. Works for me anyway....

By the way I adjust the pot to get a weaker or richer mixture as needed; I don't bother with an actual reading.

MattW · Mar 21, 2011

Trullion said:
My 850GS used to "miss" every now and again when idling and also used to run rich. I cured it by careful setting of the pot and the tps. Both the tickover and tps should be adjusted so that there is a smooth transition from tickover to main throttle. IMHO this is more important than a specific voltage for the tps setting, as long as it's in the working range. Works for me anyway....

By the way I adjust the pot to get a weaker or richer mixture as needed; I don't bother with an actual reading.

Thanks Trullion - very interesting.
However, I don't know whether I'd have the confidence to start tweaking the CO pot without a CO meter to let me know whether I was getting dangerously lean - colour me wuss

I've just fitted some new iridium plugs and I'm waiting on some Forte injector cleaner. If this doesn't help, I guess it's time for some more radical interventions

uncle dick · Mar 21, 2011

The co pot only adjusts the fuelling at idle - and maybe slightly above. To do any damage from a lean mixture it will have to be when the engine is pulling reasonably hard. This is controlled by the ecu fuel map using the various sensor inputs - temp, rpm etc.

Twiddle to your heart's content, just count the turns either way (then you can go back). It's a fine adjustment so you'll have to do a turn to notice any real difference.

Have fun,
Dick

MattW · Mar 22, 2011

uncle dick said:
The co pot only adjusts the fuelling at idle - and maybe slightly above. To do any damage from a lean mixture it will have to be when the engine is pulling reasonably hard. This is controlled by the ecu fuel map using the various sensor inputs - temp, rpm etc.

Twiddle to your heart's content, just count the turns either way (then you can go back). It's a fine adjustment so you'll have to do a turn to notice any real difference.

Have fun,
Dick

The saga continues (thanks for the ongoing help).

I fitted new iridium plugs and put a dose of fuel injector cleaner in the (full) tank after work today. Initially it felt like the idle had smoothed out a bit but this might just have been wishful thinking. I had a good thrash up the motorway where it ran fine at speed - pulled up to 100 (kph obviously :augie

) smoothly and quickly and came back home.

Ticking over at home, it felt slightly odd - fluffy and hollow sounding (if that makes any sense) - subjectively I'd have said it was even richer at idle than it was before

It was also puffing a little black smoke from the exhaust when I blipped the throttle. So I got the U tube out and checked and corrected the balance which was out slightly (new cables bedding in I guess) and checked the TPS voltage (bang on .36).

The new blance improved things slightly so I then decided to have a fiddle with the CO pot. When I turned it a turn anti clockwise, the idle speeded up a couple of hundred rpm and seemed to smooth out which I guess means it was leaning out? I stopped at this point as it had been ticking over for a while and didn't want to overheat anything but I guess I might be on to something with the CO pot?...

Thanks again to everyone who's given me advice.

M

uncle dick · Mar 22, 2011

MattW said:
When I turned it a turn anti clockwise, the idle speeded up a couple of hundred rpm and seemed to smooth out which I guess means it was leaning out? I stopped at this point as it had been ticking over for a while and didn't want to overheat anything but I guess I might be on to something with the CO pot?..

M

You've richened it up a bit which sounds like just what it needed. The colour of the tailpipe isn't a good indicator so I'd ignore that for now. Keep twiddling and let us know how you get on.

Dick

MattW · Mar 23, 2011

Just been thinking about this - I turned it clockwise (sorry - brain fade from working upside down) so I guess I leaned it. I'm assuming clockwise is lean.

If I was to take it to my local MOT station and try to sweet talk them into letting me put it on their exhaust analyser, what CO should I be aiming for? I think I recall Steptoe saying 1.5 - 2.0 % but I can't find the thread now.

Almost makes me want to buy one of these ...

DrFarkoff · Mar 23, 2011

Matt 1.5 to 2.25% is what my foreman told me at the dealers

the transition does take time to filter thru so it could be 30 seconds before you get a change
Also go and get her hot so the oil cooler is flowing oil and the ECU is on normal mode instead of cold and rich

do no more than one turn at a time and as you approach the zone go to half turns and less and give at least 30 seconds for changes to register and give it a rev every 5 mins to blow excess gases out

MattW · Mar 23, 2011

Jaythro said:
Matt 1.5 to 2.25% is what my foreman told me at the dealers

the transition does take time to filter thru so it could be 30 seconds before you get a change
Also go and get her hot so the oil cooler is flowing oil and the ECU is on normal mode instead of cold and rich

do no more than one turn at a time and as you approach the zone go to half turns and less and give at least 30 seconds for changes to register and give it a rev every 5 mins to blow excess gases out

Thanks Jaythro :thumb2

(RIP) Kaister · Mar 23, 2011

you've probably left the balance pipe nipple covers off :mmmm

i joined mine together

uncle dick · Mar 23, 2011

MattW said:
Almost makes me want to buy one of these ...

Matt,

The tool in your link is more for the re-mapping guys who want to set it up on the road - and not melt pistons. You'd never get sensible readings at idle. As Jaythro said. around 2% at idle is fine as a starting point but I had to wind mine out three turns to get a smooth ride. This wasn't uncommon with injection systems of that era.
Find a friendly Mot station - if you're in S Devon call in - and sort out your starting point. Then tweak away.

Dick

Low rev hesitation?

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Grumpy Ole Git!!!

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